When connecting an accessory, such as a headset with at least one microphone and at least one speaker, to a host device such as a mobile phone, it is generally required to transmit signals from the host device to the accessory and from the accessory to the host device.
Since many existing host and accessory devices are equipped with standard 4-pin sockets or jacks, there is an advantage in providing an interface that can use a total of four wires for the connection. Since one of the wires is used for a ground connection, there are only three signal conductors available. It is advantageous to use one of these conductors to provide power from the host device to the accessory, to avoid the need to provide a battery in the accessory, and so only two conductors are available for transferring the signals from the host device to the accessory and from the accessory to the host device.
There are known advantages in using balanced transmission, which for example reduces electromagnetic interference (EMI) emissions and improves sensitivity, but this requires the use of two wires per communication path, rather than one.
These factors mean that it may be preferable to provide a bidirectional (half duplex) link between the host device and the accessory on one wire (or on one pair of wires), rather than providing a full duplex link that would have a connection from the host device to the accessory on one wire, and from the accessory to the host device on the other wire.
One issue that arises with a bidirectional link on a single wire is the need to take account of the transmission delay between the host device and the accessory. For example, when one data symbol is transferred in a first direction from a first device to a second device, it arrives at the second device after a certain time delay. Then, if the requirement is that the next data symbol should be transmitted from the second device to the first device, there is a further transmission delay before that next data symbol is received by the first device.
In the case of a host device and an accessory, the same host device needs to be usable with different accessories, while one accessory needs to be usable with different hosts. Therefore, it is useful for the devices to be able to take account of the transmission delay that applies for a particular combination of host device and accessory.